Answer
$lookup performs cross-collection joins. Without proper indexing, it triggers large scans and increases CPU and memory usage.
Quick Answer
$lookup (join) in a sharded cluster can't be pushed down to shards if the "from" collection is on different shards - all data must come to the mongos for merging. This causes massive data movement and memory pressure. Fix: embed frequently joined data, pre-aggregate with scheduled pipelines, use Atlas Search for full document lookup, or ensure joined collections share the same shard key.
S
SugharaIQ Editorial Team
Verified Answer
This answer has been peer-reviewed by industry experts holding senior engineering roles to ensure technical accuracy and relevance for modern interview standards.
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Source: SugharaIQ